half farthest from it a positive charge. We may prove this by the pith ball already described (Art. 337), for if we charge such a pith ball positively by touching it with a stick of glass rubbed with silk, we shall find that it will be attracted by that half of the conductor which was next m, while it will be repelled by that half which was farthest from m, showing that the first half contains negative and the latter positive electricity.

It will be noticed that the electrified conductor m has not parted with any of its electricity, but is the same after the experiment as before it. Let us now, however, vary the experiment by slowly bringing the conductors nearer to one another, although not into actual contact; the attraction between the positive electricity of the charged conductor and the negaiive electricity of the neutral conductor which it has decomposed will at length become so great that they will be able to surmount the resistance of the air, and will rush together in the form of a spark. The consequence will be that the conductor m will have lost a portion of its positive electricity, and the conductor n its negative electricity which m had decomposed and attracted towards itself; a positive charge will therefore remain in ʼn : in fact, the result will be just the same as if there had been a communication of positive electricity from m to n.

The action which the charged conductor m exerts upon n at a distance is called electrical induction.

This influence is limited in its extent, and depends upon the distance between the two conductors.

To render this evident, let us suppose the charged conductor m to act upon the uncharged conductor n, separating the electricities as in Fig. III.

It is clear that the negative electricity is kept at B by the attraction of the charge at A, while this attraction has to oppose the tendency which the two separated electricities in n have to rush together and unite.

Now if the charge at m is very small, or very far away, it cannot separate a very great quantity of electricity in n, but as the distance decreases, the amount of separation will

increase, until at length, as we have already said, a spark will pass between the two conductors. The inductive effect of electricity will afford us a new explanation of the fact that electricity only shows itself at the surfaces of bodies. Thus we may regard an electrified brass globe such as that of Art. 342 as inducing an opposite electricity in the surrounding conductors, including the earth and walls of the room. The two electricities cannot however unite, because the air surrounding the globe is a non-conductor. Nevertheless the electricity of the globe will endeavour to get as near as possible to that of the surrounding exterior, and will thus appear to reside on the surface of the globe.

344. We have spoken as if the one conductor acted upon the other at a distance, but the researches of Faraday lead us to conclude that the action of induction depends upon the substance interposed between the two conductors.

If for instance the space between m and n, instead of being filled with air, had been filled with sulphur, the same charge in m would have separated a greater quantity of electricity in n.

The capacity that a substance has for causing induction when interposed between a charged and an uncharged conductor is called its inductive capacity.

LESSON XXXIX.-ELECTRICAL MACHINES, ETC.

345. An electrical machine is composed of two parts; first of all we have an arrangement for generating electricity; and secondly, we have one for collecting it.

One of the best known machines is that in which the electricity is produced by a large disc or plate of glass revolving on a horizontal axis (Fig. 112). The axis of the plate passes through wooden supports, and the handle which turns the machine is made of glass. As the glass plate revolves it is rubbed against by two set of rubbers, one above and the other below; these rubbers are generally made of leather stuffed with horse-hair, and press somewhat tightly against the glass. They are coated with an amalgam

which is generally made of one part of zinc, one of tin, and two of mercury. These rubbers are placed in electrical communication with the ground by means of a chain.

Now, when the glass disc is turned round, positive electricity is generated in the glass, and negative electricity in the rubbers, and by means of the chain the negative electricity of the rubbers is carried to the ground as fast as it is produced. Our object is to collect the positive electricity, but get rid of the negative.

Surrounding the glass we have two brass rods armed with points, these rods being metallically united to a large metallic

surface called the conductor, which is insulated by glass supports. These points act by induction on the positive electricity which we have produced by friction of the glass plate; that is to say, this positive electricity decomposes the neutral fluid of the points, attracting to itself the negative and repelling the positive into the large conductor, which is in metallic contact with the points. The positive fluid on the glass thereupon unites with the negative fluid at the points, and the result of this union is the accumulation of a quantity of positive electricity on the large conductor. (See Art. 343.)

346. If when the conductor of an electric machine is charged the finger be placed near it, a spark passes between the conductor and the finger.

The positive electricity of the conductor decomposes the neutral fluid of the body, attracting the negative and repelling the positive to the ground. The negative electricity at the point of the finger thereupon combines by means of a spark with the positive fluid in the conductor, a peculiar sensation being experienced as the combination takes place.

The experiment may be varied by placing an individual on an insulating stool, which is a stool furnished with glass legs. If he now puts himself into electrical communication by means of a chain with the conductor of the machine, he becomes part of that conductor, and another person standing on the ground can take a spark from him just as he could from the conductor itself.

Another very convenient mode of generating electricity is by means of the electrophorus.

This instrument consists of a shallow tinned pan or mould, which is filled with resin, so that at the top we have a smooth resinous surface in a mould the outside of which is metallic, and therefore in electric communication with the earth. Besides this there is a moveable metallic cover having a glass handle (see Fig. 112a), which can either be brought into contact with the resinous cake or removed from it as desired.

FIG. 112.

When about to be used the resinous surface is smartly beaten by a cat's fur, and by this means negative electricity is developed upon it. The cover is next brought into contact with the excited resin, and the upper surface of the cover is then touched with the finger. It might at first be thought that we should by this means carry off the electricity developed on the resinous surface, but such

is not the case. The resin, it must be remembered, is a non-conductor, and the cover does not come into such intimate contact with it as to carry off its electricity by conduction. Instead of this it acts inductively upon it, so that the neutral fluid of the cover is decomposed, the positive electricity being retained on the under side by the inductive action of the resin, while the negative goes away through the finger which touches the cover into the ground. The cover, when removed, is in consequence found to be charged with positive electricity.

As the electricity of the resin has not been conducted away, this operation can be repeated time after time without any apparent diminution of the electricity of the resin, and without the necessity of exciting it afresh.

The tinned mould or form in which the resin is placed serves to retain the negative electricity developed on the surface. For this electricity acts inductively on the neutral fluid of the under surface of the metallic form, retaining the positive, but driving the negative into the earth, upon which we may suppose the instrument to be placed.

This positive electricity, which thus resides on the lower surface of the form, binds in its turn the negative electricity which exists on the resinous surface, and prevents it from being easily dissipated.

This is the mode of action of the form while the cover is not on; but when the cover is put on, the electricity of the resin prefers to act inductively upon the cover which is close to it, rather than upon the bottom of the form, which is removed from it by the whole thickness of the cake, and we have then the action which has been already described.

347. Instruments for detecting Electricity.-Let us take a glass jar (Fig. 113), of which the upper and the lower part are both made of metal. Let the upper part terminate in a metal rod with a brass knob, and let this metal rod be brought inside the jar and have two bits of gold leaf or two gilded straws attached to it, hanging loosely. Also let there be inside, at a and b, two strips of gold leaf